Logging trailers have special requirements for off-road conditions. For example it is desirable for the trailer axles to travel significant vertical distances to enable a heavy payload of logs to be carried over rough terrain. Axle travel is accomplished by a pivot point where the trailer axles attach to the trailer frame. Another pivot point is found between the trailer bunk (an upright "U"-shaped frame used to receive the logs) and the trailer frame. The bunk will typically pivot about an upright axis to enable articulation of the trailer relative to the log load as the truck moves around a curve. Log trucks are thereby capable of carrying a relatively long load of logs around fairly sharp corners without the log load "cutting" through the inside of the corner. These features, plus the need for lifting the unloaded trailer onto the truck frame for transport, are at least somewhat unique to log trailers.
Some of the features provided on log trailers are intended primarily for use on narrow unpaved logging roads, where road conditions are typically not nearly as favorable as paved highways. The same features become of secondary importance during highway transport. In fact other features become desirable. For example, it is more desirable on paved roadways for loaded logging trucks to meet certain load limit requirements for each load bearing axle. Such load limitations are seldom required on unpaved logging roads.
Booster axles have been developed in the past for log truck trailers; to relieve some of the payload carried by the trailer axles and thereby meet roadway weight distribution restrictions. However, several of the prior boosters have not been totally satisfactory in both highway and logging road conditions.
At least some of the retrofit log trailer boosters require extensive modification of the original trailer frame. Retrofitting thus becomes a significantly expensive operation.
Another fairly typical problem with prior forms of log trailer load boosters is that some prior boosters do not track well behind the trailer frame, especially "steerable" boosters that are articulated about a slight off-vertical axis. It happens in certain conditions that such boosters may "fishtail" at highway speeds, creating a serious safety hazard. Also "steerable" boosters cannot be conveniently moved backwardly without frequently "jackknifing."
A still further problem with prior log trailer load boosters is discovered when the trailer is unloaded. It is very desirable to carry unloaded trailers on the truck, thereby significantly shortening the overall length of the vehicle. An unloaded truck may then be easily driven to a landing where the trailer is unloaded by a nearby log loader (if one is not provided on the truck) and hitched to the truck before loading. A trailer with a prior load booster may well be too heavy for convenient hoisting by many small capacity log loaders.
Another problem is that existing boosters will often require a special framework or "ride" that must be retrofitted to the truck chassis to receive and carry the booster when the trailer is carried on the truck. Thus the added weight of the booster and "ride" can detract from truck fuel efficiency and overall load carrying capacity.
A need has therefor been realized for a log trailer load booster that will minimize or eliminate the above problems. The present invention is intended to fill this need by providing a relatively light weight, positively tracking log trailer booster which: (a) is easily adapted to an existing log trailer without requiring extensive modification of the trailer or truck chassis; (b) will function in either of two selective modes, an inoperative, relatively freely suspended mode for operation along logging roads, so the booster will not interfere with normal handling of the trailer on logging roads, and a weight transfer mode for operation along load restricted roadways; (c) can be moved rearwardly in either mode without jackknifing; and (d) is light weight enough to be loaded onto a truck and unloaded by relatively light duty log loaders.
The above and still further objects and advantages will be better understood from the following description which, taken with the accompanying drawings, describe the best mode presently contemplated for carrying out the present invention.